ECE 480, Prof. A. Mason Sensors p.1 SENSORS a.k.a. Interfacing to the Real World: Review of Electrical Sensors and Actuators Andrew Mason Associtate Professor, ECE Teach: Microelectronics (analog & digital integrated Circ., VLSI) Biomedical Engineering (instrumentation) Research: Integrated Microsystems (on-chip sensors & circuits)
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ECE 480, Prof. A. Mason Sensors p.1
SENSORSa.k.a.
Interfacing to the Real World:Review of Electrical Sensors and Actuators
Andrew MasonAssocitate Professor, ECE
Teach: Microelectronics (analog & digital integrated Circ., VLSI)Biomedical Engineering (instrumentation)
• Thermistor Half-Bridge– voltage divider– one element varies
• Wheatstone Bridge– R3 = resistive sensor– R4 is matched to nominal value of R3– If R1 = R2, Vout-nominal = 0– Vout varies as R3 changes
VCC
R1+R4
ECE 480, Prof. A. Mason Sensors p.13
Operational Amplifiers• Properties
– open-loop gain: ideally infinite: practical values 20k-200k•high open-loop gain virtual short between + and - inputs
– input impedance: ideally infinite: CMOS opamps are close to ideal– output impedance: ideally zero: practical values 20-100Ω– zero output offset: ideally zero: practical value <1mV– gain-bandwidth product (GB): practical values ~MHz
•frequency where open-loop gain drops to 1 V/V• Commercial opamps provide many different properties
– low noise– low input current– low power– high bandwidth– low/high supply voltage– special purpose: comparator, instrumentation amplifier
ECE 480, Prof. A. Mason Sensors p.14
Basic Opamp Configuration• Voltage Comparator
– digitize input
• Voltage Follower– buffer
• Non-Inverting Amp • Inverting Amp
ECE 480, Prof. A. Mason Sensors p.15
More Opamp Configurations• Summing Amp
• Differential Amp
• Integrating Amp
• Differentiating Amp
ECE 480, Prof. A. Mason Sensors p.16
Converting Configuration• Current-to-Voltage
• Voltage-to-Current
ECE 480, Prof. A. Mason Sensors p.17
Instrumentation Amplifier• Robust differential
gain amplifier
• Input stage– high input impedance
• buffers gain stage– no common mode gain– can have differential gain
• Gain stage– differential gain, low input impedance
• Overall amplifier– amplifies only the differential component
• high common mode rejection ratio– high input impedance suitable for biopotential electrodes with high
output impedance
input stage
gain stage
⎟⎟⎠
⎞⎜⎜⎝
⎛+=
3
4
1
12d
2RR
RRRG
total differential gain
ECE 480, Prof. A. Mason Sensors p.18
Instrumentation Amplifier w/ BP Filter
instrumentation amplifierWith 776 op amps, the circuit was found to have a CMRR of 86 dB at 100 Hz and a noise level of 40 mV peak to peak at the output. The frequency response was 0.04 to 150 Hz for ±3 dB and was flat over 4 to 40 Hz. The total gain is 25 (instrument amp) x 32 (non-inverting amp) = 800.
HPF non-inverting amp
ECE 480, Prof. A. Mason Sensors p.19
Connecting Sensors to Microcontrollers
• Analog– many microcontrollers have a built-in A/D
• 8-bit to 12-bit common• many have multi-channel A/D inputs
• Digital– serial I/O
• use serial I/O port, store in memory to analyze• synchronous (with clock)
– must match byte format, stop/start bits, parity check, etc.• asynchronous (no clock): more common for comm. than data
– must match baud rate and bit width, transmission protocol, etc.– frequency encoded
• use timing port, measure pulse width or pulse frequency
µCsignal timing
memory
keypadsensor
sensor displayinstrument
ECE 480, Prof. A. Mason Sensors p.20
Connecting Smart Sensors to PC/Network• “Smart sensor” = sensor with built-in signal processing & communication
– e.g., combining a “dumb sensor” and a microcontroller• Data Acquisition Cards (DAQ)
– PC card with analog and digital I/O– interface through LabVIEW or user-generated code
• Communication Links Common for Sensors– asynchronous serial comm.
• universal asynchronous receive and transmit (UART)– 1 receive line + 1 transmit line. nodes must match baud rate & protocol
• RS232 Serial Port on PCs uses UART format (but at +/- 12V)– can buy a chip to convert from UART to RS232
– synchronous serial comm.• serial peripheral interface (SPI)